1. Field of the Invention
This invention provides improved reexpandable shrunken foam bodies of styrene-acrylonitrile type resin and a method for the manufacture thereof. Specifically, the improvement resides in the fact that the foam bodies now in a state shrunken from the former highly foamed state possess an ability to allow themselves, on standing at room temperature under atmospheric pressure, to reexpand substantially to the original volume of the highly foamed bodies. These foam bodies, therefore, may be transported in their shrunken state to a site of actual use, fed in a cavity desired to be filled with foamed bodies, and allowed to reexpand in the cavity. Or, the foam bodies may be allowed to reexpand at the site of actual use and, in the expanded state, fed into the cavity. Also, the foam bodies in the reexpanded state may be mutually bonded by fusion inside a mold to yield a highly foamed shaped article of high quality suitable for cushioning or insulation of heat.
2. Description of the Prior Art
It has been widely known that bodies of foam are obtained by causing a styrene resin to contain therein a volatile foaming agent and thermally foaming the resin. It has also been known that these bodies of foam are useful as cushioning material for filling the gap between a container and an object contained therein or as a material for a cushioning container or a heat insulation board produced by the procedure of permitting air to permeate into such bodies of foam and causing the air-filled bodies to be mutually bonded by thermal foaming in a mold of a prescribed pattern.
The techniques which are often found as desirable in this field of art include (1) the technique of producing a highly foamed body of good quality, (2) the technique of producing such a highly foamed body in a single step of foaming, and (3) the technique of enabling the foamed bodies to be transported economically to a fabricating plant (site of actual use). The reason is respectively (1) that within the range in which the foamed body is allowed to retain necessary properties intact, the amount of the resin to be used per unit volume of the foamed body decreases in proportion as the foaming of resin proceeds to a higher extent, (2) that the cost of production decreases in proportion as the number of steps involved in the operation of foaming decreases, and (3) that transportion of highly foamed bodies to a distant destination turns out to be an act as uneconomical as the act of transporting an empty container.
It is, nevertheless, widely held that these techniques earnestly sought after in the field are very difficult to realize. The reasons are as follows: (1) As the expansion ratio of the resin being foamed is increased, the individual cells in the foamed body become more liable to open into one another or to grow into nonumiformly dispersed sizes and, consequently, the produced foamed body acquires desired properties with great difficulty. (2) When the foaming of such high extent is to be effected in a single step, the maximum amount of the foaming agent allowed to be contained in the resin and the efficiency of the foaming agent directly contributing to the expansion of the resin have their limits and these limits impose serious restrictions on the operation of foaming. (3) The foamed body, once shrunken, does not readily expand to its original volume. If the shrunken body is forced to reexpand by any immoderate treatment, the reexpanded body acquires unsatisfactory properties.
An ideal foaming technique which can be hoped for reasonably from the standpoint of the technical standard prevalent today, therefore, is one which is capable of producing a highly foamed body in a single step of foaming, causing the foamed body to be shrunk to a volume small enough for convenient storage and transportion, and enabling the shrunken foamed body to be reexpanded into a highly foamed body of good quality at the time of its actual use.
A thorough review of the techniques heretofore known to the art from the aforementioned point of view reveals that a few patent publications cover inventions contemplating achievement of such techniques. The technique disclosed in the specification (specifically in working examples) of U.S. Pat. No. 3,425,965 produces partially collapsed (shrunken) foam particles by causing chlorostyrene polymer to contain therein 6.1% of a volatile organic foaming agent (isopentane), heating the polymer by contact thereof with steam of high pressure, and withdrawing the heated polymer under atmospheric pressure. These particles, when left standing under atmospheric pressure, are reexpanded into foam particles of a volume about 4.4 times (the largest value given in Table 1) the volume occupied by the shrunken particles. The technique disclosed in the above patent, as summarized above, may answer the aforementioned description of an ideal technique. Its technique, however, entails a serious problem that it is applicable only to chlorosytrene polymer, a resin scarcely available commercially. An attempt to adopt a sytrene-acrylonitrile copolymer in the place of the chlorostyrene polymer with a view to meeting the requirement for high resistance to heat and to oil imposed on the finally produced foamed article and, by the application of the technique in question to the styrene-acrylonitrile copolymer, to produce shrunken particles reexpandable at a high expansion ratio encounters the hindrance that the foamed particles obtained, at all, have their individual cells ruptured and can hardly be expected to possess the desired reexpandability.
In the specification of U.S. Pat. No. 3,505,249 there is disclosed a technique which produces reexpandable shrunken bodies of resin foam by foaming expandable thermoplastic resin particles under a vacuum, subsequently releasing the foamed resin particles under atmospheric pressure, and then collapsing them with pressure. In Example 3 cited therein, there is given a statement to the effect that styrene-acrylonitrile copolymer particles containing 7.6% by weight of n-butane were foamed under a vacuum to a 216-fold expansion, released under atmospheric pressure and consequently collapsed to a 19-fold expansion, then immediately left standing in liquid nitrogen for five minutes, and subsequently warmed to room temperature and consequently reexpanded to a 190-fold expansion. (the reexpansion wa obtained to 29-fold when the immersion in liquid nitrogen was omitted.) The problems encountered by this technique reside in the foaming under a vacuum and the reexpansion by the treatment with liquid nitrogen. To be specific, the limitations imposed on the reexpandability automatically by the amount of the foaming agent contained in the resin and the efficiency of the foaming agent allowed to contribute to the actual foaming are overcome by carrying out the foaming under a vacuum. The reexpandability which the shrunken particles have lost substantially is conferred upon the shrunken particles by having the particles immersed in liquid nitrogen. These steps of operation are so particular that their material-repeatable on a laboratory scale entail many unsolved problems as to equipment, expense, and operation on a commercial scale.
In the specification of U.S. Pat. No. 3,347,961, specifically at column 5, lines 1-25, there is a statement to the effect that foamed particles of a high expansion ratio are obtained of styrene-acrylonitrile copolymer [152-fold expansion in bulk ratio (reciprocal of bulk density)]. The method disclosed in the above patent produces uncollapsed highly foamed particles by causing particles freshly foamed to a high extent to be moved, before being collapsed by cooling, directly into a flow of hot air in circulation and allowing them to age therein for a long time. This method has not yet matured to the extent of commercialization because it takes a long time and requires use of a large apparatus. In view of the fact that highly foamed particles of styrene-acrylonitrile copolymer, once shrunk, are not readily reexpanded at room temperature and prove useless for the purpose pursued, the above patent provides one approach devoted to preventing the foamed particles from shrinkage. It does not have the technical idea of producing shrunken foamed particles capable of reexpansion.
Shrunken foam bodies of a styrene-acrylonitrile copolymer possessing an ability to allow themselves, on standing at room temperature under atmospheric pressure, to reexpand to the original volume before shrinkage at the former expansion ratio (not less than 80-fold) have never existed.
It has been held that the aforementioned reexpandable shrunken bodies of styrene-acrylonitrile copolymer foam are difficult to be fabricated from the standpoint of process of manufacture. No method has been perfected which permits their satisfactory production. Other uneconomical methods highly special in their operation have been proposed as mere makeshift.
In the field of foamed articles which, by virtue of their reexpandability, are useful for the production of heat insulation boards, cushioning materials, and buoys, the outstanding properties of styrene-acrylonitrile resin, a close analog to general purpose resins, have not been utilized commercially.